1. Field of the Invention
This invention relates to a gas sensor, in particular to a gas sensor including a metal shell welded to a metal cylinder, the welded portion having excellent corrosion resistance.
2. Description of the Related Art
A gas sensor provided with a gas detection element configured to change its electrical characteristics depending on concentration of a specific gas component in exhaust gas is used for controlling the air fuel ratio of an automobile. There are various sensors such as an oxygen sensor, an NOx sensor and a HC sensor. The gas sensor includes a gas detection element coaxially inserted into a metal shell. A leading end portion of the gas detection element protrudes from a leading end portion of the metal shell and is exposed to a gas to be measured, such as exhaust gas, so as to detect a specific gas component such as oxygen in the measurement gas.
A sensor output lead wire is connected to a base end portion of the gas detection element protruding from a base end portion of the metal shell via a pair of sensor terminal metal fittings, and a heater lead wire is connected to a ceramic heater inserted into the interior of the gas detection element from a base end side via a pair of heater terminal metal fittings. In order to protect these portions, a metal cylinder is fixed to the base end portion of the metal shell so as to surround their periphery. The metal shell and the metal cylinder are fixed by: fitting a leading end portion of the metal cylinder so as to surround a periphery of the base end portion of the metal shell; and subjecting these portions to circumferential welding. The circumferential welding forms a welded portion in which metal components of the metal cylinder are molten into the metal shell. The welded portion ensures air tightness between the metal shell and the metal cylinder.
The gas sensor is generally used under a severe environment. More specifically, the gas sensor is attached to an exhaust gas pipe of an automobile. Therefore, the gas sensor is heated to a high temperature due to heat from the exhaust gas pipe. Also, the gas sensor is attached to the exhaust gas pipe at a mounting thread portion formed at a leading end side of the metal shell. Consequently, a portion of the gas sensor is exposed to an outside of the exhaust gas pipe. The exposed portion is closer to the base end portion than the mounting thread, which includes a tool engagement portion and the metal cylinder fixed to a base end portion of the metal shell. Therefore, corrosive substances such as a snow melting agent, e.g. calcium chloride or the like, used in cold climate districts easily adheres to the exposed portion.
Since the gas sensor is used under the severe environment described above, the metal shell and the metal cylinder exposed to the outside a r e formed of stainless steel having excellent corrosion resistance. For example, the metal cylinder is formed from SUS304 having excellent corrosion resistance, and the metal shell is formed from SUS430 having cold forgeability although its corrosion resistance is slightly inferior (see JP-A-208165, for example)